This invention relates to papermachine belts, particularly to support belts for avoiding unsupported draws for example between the press and dryer sections of the machine, and more especially to planar belts having the capability of imparting a smoothing effect to the paper web in the press.
An early disclosure of a support belt arrangement in a papermachine is EP-A-107,606 (Beloit), in which it was established that the belt should be impervious to water, to avoid re-wetting the paper web, have a smooth surface, and hardness similar to a plain press roll cover.
U.S. Pat. No. 4,652,620 (Adams) discloses a papermachine belt comprising a fabric base, such as a woven scrim, which is spray coated with a urethane coating on one surface of the fabric which impregnates the fabric to a substantial depth of at least one-half of the thickness of the scrim. The coating is characterised by the presence of a myriad of small isolated closed bubbles or pores providing a stone-like closed structure. The fabric may also be coated on both surfaces and the coating may be coated with a more dense urethane coating providing a gloss finish. This belt is designed as a carrier for conveying paper through pressure nips such as in the press section of a papermachine and the finish provided by the coating enables the belt to release the paper web after the nip without damage. This belt is not envisaged as being suitable for use as a support and/or smoothing belt.
U.S. Pat. No. 4,976,821 (Laapotti) discloses a papermachine press section which features two separate successive press nips between which the web passes in a closed draw, supported by a conveying fabric. This latter is disclosed as being preferably relatively impervious and produced for example by impregnating an ordinary press felt with an appropriate plastics material, and may in some uses be to some extent pervious and/or water receiving although to a considerably lesser degree than ordinary press felts.
EP-A-576115 discloses a transfer belt wherein sheet release for transfer is assisted by incorporating particles which under pressure are compressed into the belt matrix, but on release of pressure at the web release point, stand proud of the belt surface and thus create a temporary roughening of the surface, which aids sheet release.
It is an object of the present invention to provide a papermachine belt which is capable of acting both as a draw eradication belt, and also as a medium for smoothing the surface of the paper web.
According to the invention, a papermachine belt comprises a support structure, having a layer of thermoplastic material formed on at least one face of the support structure, further fibrous and yarn-material being at least partially embedded in said thermoplastic layer.
The layer of thermoplastic material may be provided in the form of a porous membrane of material such as a thermoplastic polyurethane elastomer, which is laid on one surface of a press felt base fabric. Fibrous material in the form of non-woven batt is then placed on top of the membrane and the whole structure is then needled. The felt is then heated on the side nearest to the membrane ie. the papercontacting face, which results in the melted polyurethane passing from the inside to the outside surface through the batt.
By varying the heat, pressure and dwell time, the distribution of fibres can be controlled In some cases, a surface totally free of fibres may be desired, and in other cases, fibre in the surface may be required. This greater control arises because the membrane is within the structure and flows out to the surface, rather than flowing inwards from the outside.
The felt according to the invention may comprise a membrane carried on a woven base cloth with minimal amount of batt needled on top, and a further batt on top of the membrane, the whole structure then being needled together and then heat applied on the side of the belt nearest the membrane. Alternatively, the further batt over the membrane may be omitted.
In another embodiment, the thermoplastics material may be a cast sheet of e.g. thermoplastic elastomeric polyurethane which is introduced onto the surface of the pressfelt. Sufficient heat and pressure may then be applied to give a surface laminate structure or to give a composite whereby the material impregnates the felt, whilst maintaining impermeability.
The woven base cloth may be made by weaving bi-component yarn in either the running and or the cross-machine directions. Batt is placed over this, with the possibility of a support fabric being additionally provided.
The felt is then fused, heat being applied to the side closest to the meltable yarns.
The press felt base cloth may incorporate separately helically wound bi-component yarns.
The use of batt on top of the base fabric is advantageous as it offers frictional resistance to the yarns.
Batt is also preferably needled onto the yarns, thus effectively trapping the yarns and so preventing splaying prior to heating. This could be of particular importance if the endless felt has to be heated upside down on rollers in order for the side closest to the membrane to be in contact with the heater.
The resulting belt is preferably completely impervious to water, or of very low permeability, in the range for example 0-50 cfm at 12.5 mm WG.